1. Field of the Invention
The present invention relates to a metal welding method and metal bonding structure and, more specifically, a metal welding method and metal bonding structure by which different types of metals are bonded by, e.g., electrical resistance welding which heats and presses the metals.
2. Description of the Related Art
Conventionally, techniques which bond two different types of metals differing in, e.g., melting point by heating and pressing have been extensively used. The electrical resistance welding technique is one example of such metal bonding techniques. In this welding technique, different types of metals are brought into contact with each other and applied with an electric current and pressed in this contacted state. The two metal members are bonded by using the electrical resistance between the contact surfaces.
Japanese Patent Laid-Open No. 8-296416 has proposed a technique by which this electrical resistance welding technique is applied to bonding of a cylinder head and a valve seat. Compared to the conventional method of press fitting, this technique of bonding a cylinder head to a valve seat can decrease the thickness of the material of the valve seat and increase the diameter of an intake/exhaust port of the cylinder head. Hence, this technique greatly improves the degree of freedom of engine design.
However, it is known to those skilled in the art that the bonding surfaces of a light-alloy-based cylinder head and an Fe-based valve seat are fragile. Consequently, these bonding surfaces may be deficient in strength as bonding surfaces of these two members required to have high durability.
It is, therefore, an object of the present invention to provide a metal welding method and metal bonding structure which bond metal members differing in melting point with high bonding strength.
That is, a metal welding method of welding a first metal member (3) and a second metal member (2) having a lower melting point than that of the first metal member is characterized by comprising the steps of depositing a third metal member (7) having high reactivity to the first and second metal members on at least one of the first and second metal members, and heating and pressing the first and second metal members on at least one of which the third metal member is deposited, thereby welding the first and second metal members by a ternary bonding layer (10) containing the first to third metal members.
Also, the metal welding method is characterized in that when the first and second metal members are welded, the third metal member is discharged from the contact portion of the first and second metal members, and the ternary bonding layer is formed in the contact portion.
To achieve the above object, a metal bonding structure according to the present invention is characterized in that a first metal member (3) and a second metal member (2) having a melting point lower than that of the first metal member are bonded via a ternary bonding layer (10) containing the first and second metal members and a third metal member (7) having high reactivity to the first and second metal members.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.